Both $\ce{NH3}$ and $\ce{NF3}$ molecules formed as the shape of pyramid (bonds pairs-3 lone pairs-1) But the angles between those molecules different to the each other because of the electronegativity of the each atom.
Electronegativity of each atom
Nitrogen-3.07
Hydrogen-2.1
Florin-3.98
Get the molecule $\ce{NH3}$. Actually according to the Folin's law nitrogen's electonegativity is more than the Hydrogen. Therefore the bond electron cloud tries to attract to the direction of the nitrogen atom. There for the whole electron cloud attract to the central atom. Then the each electron pair come closer to the each and because of that electrons tries to repeal.(When electrons come closer to the each other they try to repeal) And then the the angles been increased because of the repultion.
Then get the molecule $\ce{NF3}$. It also making a pyramidal shape. The electronagitivity of Florin is more than the Nitrogen. Therefore the bond pairs attract to the florin atoms, that means they attract to the terminal atoms. Then the distance between electon pairs increase, and not try to repulsion. Because of thar the angles be smaller.
*If the repulsion is very large electrons try to go far. Then the angle increase.*If the repulsion is small electrons not try to go far. Then the angle is try to be bended.
Therefore the bond angle of $\ce{NH3}$ is more than $\ce{NF3}$.
Now think about the angle of $\ce{NCl3}$ also.